1. Field of the Invention
The present invention relates to a method of manufacturing a metallic tube with spiral fin, used as for example a heat transfer tube of a heat exchanger and the like, in particular to a method of manufacturing a metallic tube with spiral fin directly from a solid metallic bar or a metallic elementary hollow bar by the use of a piercing mill of inclined type.
2. Description of the Prior Art
A metallic tube with spiral fin has been used as a heat transfer tube of a heat exchanger and the like and various methods of manufacturing such a metallic tube with spiral fin have been proposed. The main methods include a first method, in which a spiral fin is spirally fixed to an outer circumference of a metallic tube by welding (Japanese Patent Application Laid-Open No. 9715/1983), a second method, in which a fin is spirally formed on an outer circumference of a metallic tube by thread rolling (JOURNAL OF THE JAPAN SOCIETY FOR TECHNOLOGY OF PLASTICITY, Vol. 10, No. 105, 1969-10, pp 731-732) and the like.
The first method is, as shown in FIG. 1 (perspective view), a method, in which a belt-like plate material 32 is continuously supplied, so that one edge thereof may be engaged with an outer circumferential surface of a cylindrical metallic tube 31 transferred in a longitudinal direction while rotating around an axis thereof, to spirally wind said belt-like plate material 32 on the outer circumferential surface and a joining portion of the metallic tube 31 and the belt-like plate material 32 is subjected to the high-frequency welding.
In the case where a metallic tube with spiral fin is manufactured by such a method, various disadvantages have occurred in that for example a great driving force is required for winding the belt-like plate material 32 around the outer circumferential surface of the cylindrical metallic tube; increasing a speed of winding the belt-like plate material is difficult; a tensile stress is apt to generate on the outer circumferential side of the belt-like plate material 32 wound around the metallic tube 31 and cause cracking while a compression stress is apt to generate on an inner circumferential side of the belt-like plate material 32 wound around the metallic tube 31 and produce folds, plaits and waving; complete welding of the joining portion of the metallic tube 31 and the belt-like plate material 32 is difficult; in the case where the obtained metallic tube with spiral fin is used as a heat transfer tube, any incomplete welding of the joining portion leads to inferior heat transfer characteristics; and portions, which are incompletely welded, are separated after a long-term use as boiler tubes and the like.
On the other hand, the second method is, as shown in FIG. 2 (front view) and FIG. 3 (sectional view of FIG. 2 taken along the line III--III thereof), a method, in which an elementary tube T.sub.O with a mandrel inserted therethrough is subjected to cold (or hot) thread rolling by means of three thread rolls 41, 42 and 43 disposed around a pass line of the elementary tube T.sub.O. The rolls 41, 42 and 43 are the same in shape and consist of several tens of disk rolls, which are thinwalled disks and have a sectional shape of an outer circumferential portion thereof thinned in the form of a wedge, an almost U-letter shaped groove being formed on an outer surface of the elementary tube T.sub.O by a rolling pressure in the direction of wall-thickness, the groove being rolled by the following disk roll with increasing the depth by a force acting in a direction meeting at right angles with a surface of the groove, and an amount of metal pushed aside by the plastic working of the groove being deformed in a gap between the disk rolls of the above described rolls to form a fin 44. The elongation of the tube in an axial direction thereof hardly occurs during this process and also an inside diameter of the tube being maintained at almost the same level as that of the elementary tube until the process is over.
However, in the above described second method, materials of a heat transfer tube, which can be manufactured, are limited to soft metals, remarkably superior in workability, such as Al and Cu. It is difficult to form a spiral fin made of high-alloy steel, stainless steel and standard steel and the application of this method is limited to the rolling under the condition that the elongation of the tube itself is hardly produced.
In order to eliminate the above described disadvantages, the present inventors have proposed a third method, in which a roll comprises annular grooves formed on an outer circumferential surface thereof so that their intervals may be gradually widened toward an outlet side from an inlet side of materials, a mandrel bar being inserted into an elementary tube, and the elementary tube with the mandrel bar inserted thereinto being rolled in an elongated manner by means of a rolling mill of inclined type to obtain spiral fin by thread rolling (Japanese Patent Appln. Laid-Open No. 124023/1987).
That is to say, as shown in FIG. 4 (front view) and FIG. 5 (enlarged side view showing a section of FIG. 4 taken along the line V--V thereof), a cross-type inclined rolling mill provided with three (or four) rolls 61, 62, 63 disposed around a pass line, a plurality of annular grooves for forming a spiral fin is formed in a circumferential direction of the outer circumferential surface thereof so that their intervals may be gradually increased toward the outlet side from the inlet side of a metallic tube, is used, a hot hollow metallic elementary tube T.sub.O being supplied among said rolls 61, 62, 63 with inserting a mandrel 66 into the hollow portion to carry out the inclined rolling of said metallic elementary tube T.sub.O, thereby forming a spiral fin.
According to such third method, the spiral fin can be formed on an outer circumferential surface of also the metallic elementary tube made of standard steel, stainless steel and the like.
However, in the case where a metallic tube with spiral fin is manufactured according to the above described third method, material of the metallic elementary tube pushed aside by rolls flows mainly in the axial direction of the metallic elementary tube and the metallic elementary tube is elongated in the axial direction thereof while forming a fin during the process of deforming the metallic elementary tube.
Accordingly, a problem has occurred in that in the case where a metallic tube with spiral fin having a high fin (hereinafter referred to as high-fin tube) requiring the flow of a material thereof in a radial direction thereof is manufactured, the above described method can not be effectively used.
Incidentally, in the case where a high fin tube made of standard steel and stainless steel having a ductility at hot rolling temperatures which is lower than that of soft metals, such as Cu and Al, is manufactured by the above described method, it is required to use a hollow metallic elementary tube having an outside diameter corresponding to an outside diameter of the fin and an inside diameter slightly larger than that of the product. Consequently, it is necessary to use a thick-walled tube having a wall-thickness to outside diameter ratio exceeding for example 30%. In the case where the above described method is practiced using such a metallic elementary tube having a large outside diameter, an energy required for the plastic working in the inclined rolling is increased and at present a thick-walled tube having a wall-thickness to outside diameter ratio exceeding 30% can not be manufactured into a seamless tube at any existing mandrel mill plant, so that for example a hole must be pierced by mechanical working in using a drill, for which a large amount of time is required, being required for the internal drilling of the metallic elementary tube, yield o the material being reduced, and the like, and as a result, the cost of production is remarkably increased. Accordingly, the above described method has not been applied but usually the conventional first method shown in FIG. 1 has been applied.
A metallic tube with spiral fin, in which the fin does not stand vertically relatively to an axis of tube, has been known. Since two pieces of such a metallic tube with spiral fin (hereinafter referred to as screw tube) disposed in parallel are rotated to crush solid substances put between the fins according to various circumstances, such a metallic tube with fin can be used for garbage disposal facilities, the crushing of scraps, soils and sands and the like.
In the case where the screw tube is manufactured by the first method, a metallic plate for the fin is welded while it is obliquely pressed against the tube. But, since the metallic plate is pressed against the tube and welded to the tube at the same time, as understood from FIG. 1, the metallic plate is bent or the welded portion can not be satisfactorily fixed, as above described. Furthermore, since the metallic plate is obliquely welded, difficult points have occurred in that it is difficult to position the metallic plate and both sides of the metallic plate are different in strain, so that bending or cracking is still more easily produced which remarkably reduces the production speed of the screw tube.